1. Field of the Invention
The present invention generally relates to image forming apparatuses and control methods thereof, and particularly relates to electrophotographic image forming apparatuses.
2. Description of the Related Art
Generally, APC (automatic power control), which makes uniform the amount of laser light during a single scan, is employed in image forming apparatuses in order to make uniform the density of images. However, even though control can be achieved to keep the amount of laser light uniform, this alone does not eliminate an effect of charging unevenness that occurs in the surface electric potential of a photosensitive drum. Charging unevenness refers to a phenomenon in which a charging electric potential does not become constant in a photosensitive drum that employs a-Si or the like as an image carrier. If this charging unevenness exceeds an allowable range, density unevenness occurs in the image that is formed. Japanese Patent Laid-Open No. 2005-66827 proposes a technique for reducing charging unevenness with respect to a main-scanning direction.
With the invention described above, there is an advantage in that the effect of charging unevenness in the main-scanning direction of the photosensitive drum can be reduced. However, in practice, charging unevenness can also occur in a sub-scanning direction. Accordingly, it is necessary to reduce the charging unevenness in the sub-scanning direction also in order to achieve further increases in image quality.
On the other hand, a storage unit such as a memory or the like is necessary in order to hold information relating to charging unevenness. Unfortunately, in order to store information concerning charging unevenness for not only the main-scanning direction but also the sub-scanning direction, a relatively large-capacity memory becomes necessary.